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Polymer Treatment of Oil Sands Tailings: Experimental and Modeling Investigations
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- Author / Creator
- Vajihinejad, Vahid
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Near one billion cubic meters tailings have been released to the environment as the result of oil sands mining operations in Canada. These fluid fine tailings are alkaline slurries of water, clays, and residual bitumen that after 3 to 5 years of gradual sedimentation form a thick mud-like slurry (30-40% by weight solid) called mature fine tailing (MFT). MFT clays do not settle by gravity even after decades. Polymer flocculants are used to dewater tailings by aggregating these micron-sized particles. Typical flocculants are high molecular weight anionic polyacrylamides, A-poly(AAm). A-poly(AAm) can capture micron-sized clay particles in aggregates having characteristic dimensions up to several hundred micrometers. However, residual bitumen and process water ions in oil sands tailings affect the interactions between this flocculant and clays, and reduce the flocculant performance. Calcium ions are often used in combination with A-poly(AAm) to reduce the charge density of the MFT clays. This combination forms aggregates that settle fast and generates a reasonably clean supernatant, but it also produces a gel-like sediment that is hard to dewater to the required solids content levels. Copolymers of acrylamide and partially hydrophobic or cationic comonomers have also been used to treat MFT, some with promising dewatering performance. However, what is missing in the field of oil sand tailings treatment is a more fundamental understanding on how the microstructure of the polymer flocculant affects its flocculation and dewatering performance. In this thesis, a combination of polymer reaction engineering tools and design of experiments were used to synthesize polymer flocculants
to investigate the impact of flocculant microstructure in dewatering MFT.The results of this thesis comprise three studies:
1- Synthesis of acrylamide and diallyl dimethyl ammonium chloride (DADMAC) copolymer flocculants with different properties. The results showed that the dewatering capacity of aggregates, as measured by capillary suction time (CST) and specific resistance to filtration (SRF), was not a strong function of molecular weight of the copolymers, but was substantially influenced by their chemical composition. The settling rate of the aggregates and the clarity of the supernatant, on the other hand, depended on both average molecular weight and chemical composition of the copolymer flocculant. A similar cationic copolymer was also used to examine the effect of its chemical composition distribution (CCD) on MFT dewatering. The results showed that the performance of flocculants with narrow and broad CCD is dependent on copolymer dosage: at low dosages, polymers with broad CCD form larger aggregates that settle faster and dewater more quickly, while at higher dosages a shift in performance was observed, where polymers with narrow CCD produced larger aggregates.
2- Synthesis of a novel homopolymer flocculant that met the required metrics of tailings dewatering and to address the challenges associated with the production of copolymer flocculants. The high molecular weight cationic homopolymer of (vinylbenzyl) trimethylammonium chloride, poly(VBTMAC) showed excellent performance in dewatering of high solids MFT as measured by filtration tests. The modes of flocculation of poly(VBTMAC) were found to be a combination of charge neutralization and bridging, strongly influenced by chain relaxation on the surface of particles. We also tested the ultimate potential of poly(VBTMAC) in dewatering undiluted MFT. Poly(VBTMAC) showed to be a superior flocculant compared to the industry standard, high molecular weight A-poly(AAm), consistently producing more compact aggregate network with improved dewatering rate and sediments of higher shear
strength.3- To further investigate the flocculation and kinetics of MFT aggregate formation by poly(VBTMAC), population balance models were developed using experimental data obtained by focused beam reflectance measurement (FBRM) to describe the flocculation kinetics and predict the MFT aggregate sizes obtained in different conditions. A time-varying function was defined to account for the aggregate size evolution trends observed during flocculation. The validity of the model was tested by varying shear rates, mixing time, and polymer dosage using FBRM measurements. The proposed model is the first of its kind towards a more rational and quantitative approach to control treatment processes for oil sands tailings.
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- Graduation date
- Spring 2020
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.